1. Field of the Invention
The present invention relates to a sound-absorbing body and especially relates to a thin sound-absorbing body which has excellent sound absorption characteristics with regard to a low tone range.
Priority is claimed on Japanese Patent Application No. 2006-284028, filed Oct. 18, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
In conventional cases, there is a well-known sound-absorbing material which provides a backside airspace at the backside of a sheet made from a fiber material such as glass wool, a porous material or a vibration-damping constituent including a resin.
If the sound-absorbing material provides the sheet which is made from the fiber material or the porous material, there is a tendency in which the sound-absorbing material has less sound-absorbing characteristics if a frequency is lower. Therefore, in order to improve the sound-absorbing characteristics with regard to a low frequency band, it is necessary to increase the thickness of the sheet made from the fiber material or the porous material, and it is necessary to provide the backside airspace so as to have a sufficient thickness.
On the other hand, with regard to a sound-absorbing material which provides a resin sheet made from a vibration-damping constituent including a resin such as described in Patent Document 1 (Japanese Patent Application, First Publication No. 2006-52377), it is well-known that, if an air vibration caused by a sound is applied on a front side surface of the resin sheet which is made from the vibration-damping material including a resin, a first mode vibration is caused on the resin sheet itself. A frequency caused by the first mode vibration at a sound absorption peak is determined based on a rigidity of the resin sheet and a ratio of the thickness between the resin sheet and the backside airspace. Therefore, for example, if it is required to absorb sounds which have comparatively lower frequency, it is considered to be necessary to provide the backside airspace of a certain thickness or thicker.
Because of the above-described problems, if it is required to effectively absorb sounds of 500 Hz or lower by using the conventional sound-absorbing material, it is necessary to provide a considerably thick backside airspace. Therefore, there is a problem in achieving a sound-absorbing material which is thin and which has improved sound absorption characteristics with regard to a lower frequency band.
On the other hand, in conventional cases, a normal incidence sound-absorption coefficient is generally used as a measurement for evaluation when a sound-absorbing material is designed. However, in a practical case, sounds of random incidence hit the surface when the sound absorbing material is used.
Therefore, there is another problem in which an evaluation by using the normal incidence sound-absorption coefficient is not sufficient for designing the sound-absorbing material.